This invention relates to embedding information in an image.
Conventionally, hand-written signatures, seals, or other markings are used by artists and authors to identify documents or images as their work, for example to claim a copyright or other ownership right on their work. However, digital technology for manipulating images has made it difficult to mark images in a way that the marking cannot be removed or obliterated. Additionally, owing to the popularity of Internet, the use and transfer of digitized media including media bearing digitized images has increased. Therefore, it is imperative to protect works from intentional or unwitting use which is contrary to an owner""s rights. A commonly used method for identifying a work is to insert a watermark into the original work. Watermarks which are embedded in an original work are expected to tolerate attacks of any kind. Detection of a valid watermark in a work enables an owner of the work to identify the work as their own. It is desirable to be able to detect such a watermark, even if the work is modified, for example by processing an image.
The invention is directed to a novel image protection scheme named xe2x80x9ccocktail watermarkingxe2x80x9d. To improve over current spread-spectrum watermarking approaches, two watermarks, which play complementary roles, are simultaneously embedded into an original image. The new watermarking scheme has the characteristic that, no matter what an attack is, at least one watermark typically survives well and can be detected. Results of extensive experiments indicate that our cocktail watermarking scheme is effective in resisting various attacks.
In one aspect, in general, the invention is a method for adding information to a first image including the following steps. The method includes transforming the first image to form a set of transform coefficients which represent the image. A first subset of the transform coefficients is selected and each of this first subset is modified such that the magnitude of each of the coefficients more likely to be increased than decreased. A second subset of the transform coefficients is selected and modified such that the magnitude of each of the coefficients is more likely to be decreased than increased. The method then includes forming a second image using the modified first and second subsets of transform coefficients.
The invention can include one or more of the following features:
Transforming the first image is done by computing a wavelet transform of the image, and the second image is formed by taking an inverse wavelet transform of modified wavelet transform coefficients.
The magnitude of each of the coefficients in the first and the second subsets is greater than a just noticeable difference value for that coefficient.
Modifying each of the first subset of transform coefficients includes increasing the magnitude of each of said first set of coefficients, and modifying each of the second subset of transform coefficients includes decreasing the magnitude of each of said second set of coefficients.
The method further includes computing a set of random numbers. Increasing the magnitude of each of the first subset of coefficients then includes increasing the magnitude of each of the coefficients according to a different one of the random numbers, and decreasing the magnitude of each of the second subset of coefficients includes decreasing the magnitude of each of the coefficients according to a different one of the random numbers.
The method can further include accepting a third image, which may be a processed version of the second image. The method then includes transforming the third image to form a set of transform coefficients which represent said third image and computing a difference between the transform coefficients of the first image and the transform coefficients of the third image. An indicator that the third image is a processed version of the second image is then determined from the computed difference.
In another general aspect of the invention, a method for detecting information embedded in an image includes the following. The method includes accepting an image and transforming the accepted image to form a set of transform coefficient which represent the accepted image. The method also includes accepting an original image and transforming the original image to form a set of transform coefficients which represent the original image. A difference between the transform coefficients of the original image and the transform coefficients of the accepted image are computed. Multiple estimates of a watermark sequence are determined such that each estimate is determined from a different subset of the computed differences between transform coefficients. Multiple indicators that the watermark sequence was encoded in the accepted image are computed, each indicator being associated with a different one of the determined estimates of the watermark sequence. The method then includes determining an overall indicator that the watermark sequence was encoded in the accepted image from the plurality of indicators.